Chemical and Physical Interpretation of Mdi Cure in Saturated Steam Environments

The cure of polymeric 4-4’ diphenyhnethane diisocyanate, PMDI, in wood composite manufacturing has been the subject ‘of much research. The exact .contribution of polyurethane, polyurea, and polyurete formation to PMDI/wood bonding is still debated. This study foregoes the mechanism controversy and studies the cure from a panel consolidation process. Micro-dielectric analysis, JJDEA, was ut i l ized to monitor the cure of PMDI in a controlled environment of heat, steam, and pressure simulating those encountered during wood composite manufacturing. A small steam-generating chamber was mounted to a universal test ing machine that produced saturated steam environments between 110” and 140°C. The degree of conversion calculated from pDEA provided a basis for further spectroscopic, calorimetric, and lap-shear analysis. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) revealed a large consumption of isocyanate early in cure. However, lap-shear analysis showed that mechanical strength did not develop until late in cure. Low ultimate lap-shear strengths and a plateau in conversion rates were detected for bondlines cured at 110” and 120°C. These characteristics may indicate a transition to diffusion-controlled reaction resulting from a vitrification effect such as crystallization. A phenomenological approach to composite cure kinetics was applied to model isothermal pDEA and dynamic DSC data. Models successfully predicted cure that followed analytical results. Higher activation energies were obtained for pDEA than were generated from DSC methods. The observed differences in activation energy are interpreted in terms of differing mechanisms in the progression of chemical and physical cure.